Electronic systems have long been known for measuring range of a target object, such as the various radar systems well known in the art. These systems most commonly operate on the principle of first transmitting electromagnetic energy to the object, detecting a return wave of electromagnetic energy reflected from the object, and then measuring the elapsed time it takes for the wave to have traversed the distance to the object and back. This time is then related to the distance to the object, given the known propagation rate of the wave.
Yet another such system illuminated the target with a continuous wave radar signal. A phase comparison circuit compared the phases of the transmitted and received waves and thence generated a phase difference signal therefrom as being indicative of the range to the object. A general discussion of such radar phase interferrometry methods of range detection may be found in Introduction to Radar Systems, McGraw Hill Book Co., 1962, by M. Skolnik, which is incorporated by reference.
One problem associated with these systems was that the reflected wave frequently included spurious harmonic distortion and interference or background "clutter" arising from such things as the non-linear diode action in metallic junctions of man-made objects, such objects which generated and transmitted multiples of the transmitter frequency to the receiver. This rendered it difficult to detect the information portion of the received signal attributable to the transmitted signal.
Accordingly, ranging systems designers began to seek way to reduce this problem as, for example, by improving the strength of the reflected signal. This was accomplished by various means including the provision of signal reflectors and transponder means for actively re-transmitting the received signal from the object.
A representative such system which illustrates some of the disadvantages of the prior art may be seen disclosed in U.S. Pat. No. 4,170,773 to Fitzsimmons, et al. In this system, a transponder at the target was provided for actively reflecting the transmitted wave. This required local oscillator, mixer, and phase lock loop circuitry as well as a source of power. Such an approach resulted in associated problems of limited lifetime and reliability due to notorious high failure rate of amplifiers, oscillators, and the like which were called for, limited portability and flexibility in terms of operating frequency, and higher costs due to the number of components required.
Moreover, such a system did not provide any mechanism for operation of the transponder in a phasecoherent frequency divide-down mode, but rather employed the aforementioned mixers and the like, giving rise to problems of inexact frequency division and control of the local oscillator frequency and phase offset.
Thus, a distance measurement system was highly sought after which was extremely accurate, easily scaleable to a variety of frequencies, relatively immune to harmonic distortion and interference, particularly with respect to integer multiples of the transmitted frequency, and wherein the transponder was of an inexpensive, light, simple, reliable, and easily-fabricated construction requiring no active power sources, phase lock loops, local oscillators, mixers, amplifiers, or the like.
These and other disadvantages of the prior art are overcome by the present invention.